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Relationship between biological responsiveness to phorbol esters and receptor levels in GH4C1 rat pituitary cells
Abstract:
During a 24-hr incubation of GH4C1 cells with phorbol esters or thyrotropin-releasing hormone, there is a decrease in the number of phorbol esters receptors (down-modulation). The purpose of this study was to investigate whether this decrease in receptor number attenuated cellular responsiveness to subsequent challenge with phorbol esters. Accordingly, cellular sensitivity to a phorbol ester-mediated biological response, namely the decrease in binding of epidermal growth factor, was compared in control and down-modulated cells. This phorbol ester-mediated event is closely associated with phorbol ester receptor occupancy, and it is therefore an effect for which altered dose-response characteristics correlating with alterations in the number of phorbol ester receptors could be anticipated. In fact, during a 48-hr exposure to phorbol esters, GH4C1 cells become refractory to the effect on epidermal growth factor binding. This time course is similar to that for the loss of phorbol ester receptors. However, when cells are down modulated by pretreatment with phorbol ester or thyrotropin-releasing hormone and then (re)challenged with phorbol ester, no differences in dose-response characteristics were observed between control and down-modulated cells. We therefore conclude that phorbol ester receptor down-modulation does not affect cellular responsiveness to phorbol esters, at least when decreased epidermal growth factor binding is used as the marker for the phorbol ester-mediated event.
Insights
Phorbol ester receptor down-modulation in GH4C1 cells does not alter cellular responsiveness to phorbol esters. This finding suggests receptor number changes do not impact biological responses, using epidermal growth factor binding as a marker.
Area of Science:
- Cellular biology
- Molecular endocrinology
Background:
- GH4C1 cells exhibit decreased phorbol ester receptors upon 24-hr incubation with phorbol esters or thyrotropin-releasing hormone.
- This phenomenon, known as receptor down-modulation, is a critical cellular response to specific stimuli.
Purpose of the Study:
- To determine if reduced phorbol ester receptor numbers affect cellular sensitivity to subsequent phorbol ester challenges.
- To investigate the relationship between receptor density and cellular response magnitude.
Main Methods:
- Comparing cellular sensitivity in control versus down-modulated GH4C1 cells.
- Measuring the phorbol ester-mediated decrease in epidermal growth factor binding as a biological response marker.
- Assessing dose-response characteristics after receptor down-modulation.
Main Results:
- GH4C1 cells exposed to phorbol esters for 48 hours become refractory to epidermal growth factor binding reduction, mirroring receptor loss.
- Despite receptor down-modulation, no significant differences in dose-response characteristics were observed between control and pre-treated cells upon re-challenge.
- Cellular responsiveness, indicated by epidermal growth factor binding, remained unchanged despite altered phorbol ester receptor numbers.
Conclusions:
- Phorbol ester receptor down-modulation does not impair cellular responsiveness to phorbol esters.
- The number of phorbol ester receptors does not dictate the cellular sensitivity to phorbol ester-induced biological events, specifically the modulation of epidermal growth factor binding.